Actinide Coordination Research Articles

Exploring the solution behavior of f-element coordination compounds: a case study on some trivalent rare earth and plutonium complexes

Several rare earth coordination compounds and the first actinide coordination compound of the recently introduced multifunctional ligand (S)P[N(Me)NC(H)Py]3 (1, Py = pyridyl) have been synthesized and characterized. The electronic and structural properties of these complexes were probed by X-ray diffraction analysis, X-ray absorption fine structure (XAFS), and advanced nuclear magnetic resonance (NMR) spectroscopy. Pulsed field-gradient spin-echo (PGSE) diffusion measurements and 1H,19F heteronuclear Overhauser spectroscopy (HOESY) revealed that the degree of ion pairing of the trivalent rare earth complexes [Ln(1)(OTf)3] (Ln = Y (2), La (3), Sm (4), and Lu (5); [OTf]− = [O3SCF3]−) depends on their metal cation ionic radii and decreases in acetonitrile solution for the smaller lanthanides. The plutonium(III) complex 6 exhibits, however, a significantly different behavior in solution and has a much stronger tendency to form solvent-separated ion pairs.

Single Crystal Synthesis Methods Dedicated to Structural Investigations of Very Low Solubility Mixed-Actinide Oxalate Coordination Polymers

Two crystal growth methods dedicated to very low solubility actinide coordination polymers have been developed and applied to the synthesis of mixed actinide(IV)–actinide(IV) or actinide(IV)–actinide(III) oxalate single crystals of a size (typically 100–300 μm) suitable for isolating them and examining their crystal structure. These methods have been optimized on mixed systems composed of U(IV) and lanthanide (surrogate of trivalent actinides) and then assessed on U(IV)–Am(III), Pu(IV)–Am(III), and U(IV)–Pu(IV) mixtures. Three types of single crystals characterized by different structures have been obtained according to the synthesis and the chemical conditions. This is the first time that these well-known or recently discovered key compounds are formed by crystal growth methods specifically developed for actinide crystal handling (i.e., in glove boxes), thus enabling direct structural studies on transuranium element systems and acquisition of basic data. Characterization by X-ray diffraction, UV–visible ...

Actinide Complexation with Biomimetic Phosphorylated Molecules

ABSTRACTMost data available on the interaction of actinides with biological systems are based on physiological or biokinetic measurements, with scarce information on the structure of the actinide coordination site. This proceeding article describes an approach for structural elucidation of actinide biological complexes. Indeed most of c.a. actinide circulation pathways are unknown, as they accumulate mostly in bones, kidney and liver. In case of accidental release of radionuclide in the environment, internal contamination under either acute or chronic conditions has the potential to induce both radiological and chemical toxicity through significant interaction with the metabolome or proteome followed by possible functional modifications. For instance, the metalloproteins present primary, secondary and tertiary structures, and also different post-translational modifications, all playing a crucial role in interacting with their partners, which can be altered by actinide bonding. When tightly bound, metal ions are critical to the function, structure, and stability of the proteins, by disabling specific interactions through significant local or global conformational modifications. In order to overcome the intricacy of actinide chemistry combined with that of metalloproteins, a simplified study toward better understanding the interaction of actinides and biological systems using simple biomolecules such as amino acids has therefore been considered. Focus is made on the cation coordination site itself, given that conformational effects are not taken into account in this approach. In a first step, we have selected simple phosphorylated building blocks that may be considered as chemical representatives of some ubiquitous target metalloproteins or some important phosphorylated peptides or proteins.

Complexation of Lanthanides(III), Americium(III), and Uranium(VI) with Bitopic N,O Ligands: an Experimental and Theoretical Study

New functionalized terpyridine-diamide ligands were recently developed for the group actinide separation by solvent extraction. In order to acquire a better understanding of their coordination mode in solution, protonation and complexation of lanthanides(III), americium(III), and uranium(VI) with these bitopic N,O-bearing ligands were studied in homogeneous methanol/water conditions by experimental and theoretical approaches. UV-visible spectrophotometry was used to determine the protonation and stability constants of te-tpyda and dedp-tpyda. The conformations of free and protonated forms of te-tpyda were investigated using NMR and theoretical calculations. The introduction of amide functional groups on the terpyridine moiety improved the extracting properties of these new ligands by lowering their basicity and enhancing the stability of the corresponding 1:1 complexes with lanthanides(III). Coordination of these ligands was studied by density functional theory and molecular dynamics calculations, especially to evaluate potential participation of hard oxygen and soft nitrogen atoms in actinide coordination and to correlate with their affinity and selectivity. Two predominant inner-sphere coordination modes were found from the calculations: one mode where the cation is coordinated by the nitrogen atoms of the cavity and by the amide oxygen atoms and the other mode where the cation is only coordinated by the two amide oxygen atoms and by solvent molecules. Further simulations and analysis of UV-visible spectra using both coordination modes indicate that inner-sphere coordination with direct complexation of the three nitrogen and two oxygen atoms to the cation leads to the most likely species in a methanol/water solution.

ChemInform Abstract: Advances in Actinide Solid‐State and Coordination Chemistry

AbstractReview: 74 refs.

Ionic Liquids for Lanthanide and Actinide Chemistry

AbstractIonic liquid chemistry has developed into a tremendously popular field of chemistry in the last decade. It has been realized that ionic liquids show features that cannot be realized in conventional molecular solvents. The widely tuneable class of ionic liquids has become important for lanthanide and actinide coordination chemistry, f‐element spectroscopy, f‐element electrochemistry and electrodeposition, organic synthesis and catalysis as well as inorganic nanomaterial synthesis. For example, ionic liquids offer the possibility to synthesize and crystallize quite uncommon lanthanide compounds. Ionic liquids can also be designed in such a way that they become excellent media to study the optical properties of lanthanide compounds. And even ionic liquids based on f‐elements can be made. Hydrophobic ionic liquids may replace organic solvents in liquid/liquid f‐element extraction processes, not only as a safer extraction phase but also as a smart extractant. Their wide electrochemical window and large ion conductivity make ionic liquids interesting solvents to study the electrodeposition of the very electropositive lanthanide elements. For f‐element‐catalyzed reactions, higher reaction rates, enhanced selectivity, better immobilization of the catalyst and an easy product recovery have often been observed in certain ionic liquids.

Molecular solids of actinide hexacyanoferrate: Structure and bonding

The hexacyanometallate family is well known in transition metal chemistry because the remarkable electronic delocalization along the metal-cyano-metal bond can be tuned in order to design systems that undergo a reversible and controlled change of their physical properties. We have been working for few years on the description of the molecular and electronic structure of materials formed with [Fe(CN)6]n- building blocks and actinide ions (An = Th, U, Np, Pu, Am) and have compared these new materials to those obtained with lanthanide cations at oxidation state +III. In order to evaluate the influence of the actinide coordination polyhedron on the three-dimensional molecular structure, both atomic number and formal oxidation state have been varied : oxidation states +III, +IV. EXAFS at both iron K edge and actinide LIII edge is the dedicated structural probe to obtain structural information on these systems. Data at both edges have been combined to obtain a three-dimensional model. In addition, qualitative electronic information has been gathered with two spectroscopic tools : UV-Near IR spectrophotometry and low energy XANES data that can probe each atom of the structural unit : Fe, C, N and An. Coupling these spectroscopic tools to theoretical calculations will lead in the future to a better description of bonding in these molecular solids. Of primary interest is the actinide cation ability to form ionic — covalent bonding as 5f orbitals are being filled by modification of oxidation state and/or atomic number.

The Role of Transferrin in Actinide(IV) Uptake: Comparison with Iron(III)

The impact of actinides on living organisms has been the subject of numerous studies since the 1950s. From a general point of view, these studies show that actinides are chemical poisons as well as radiological hazards. Actinides in plasma are assumed to be mainly complexed to transferrin, the iron carrier protein. This paper casts light on the uptake of actinides(IV) (thorium, neptunium, plutonium) by transferrin, focusing on the pH dependence of the interaction and on a molecular description of the cation binding site in the protein. Their behavior is compared with that of iron(III), the endogenous transferrin cation, from a structural point of view. Complementary spectroscopic techniques (UV/Vis spectrophotometry, microfiltration coupled with gamma spectrometry, and X-ray absorption fine structure) have been combined in order to propose a structural model for the actinide-binding site in transferrin. Comparison of our results with data available on holotransferrin suggests some similarities between the behavior of Fe(III) and Np(IV)/Pu(IV)/ Np(IV) is not complexed at pH <7, whereas at pH approximately 7.4 complexation can be regarded as quantitative. This pH effect is consistent with the in vivo transferrin "cycle". Pu(IV) also appears to be quantitatively bound by apotransferrin at around pH approximately 7.5, whereas Th(IV) was never complexed under our experimental conditions. EXAFS data at the actinide edge have allowed a structural model of the actinide binding site to be elaborated: at least one tyrosine residue could participate in the actinide coordination sphere (two for iron), forming a mixed hydroxo-transferrin complex in which actinides are bound with transferrin both through An-tyrosine and through An--OH bonds. A description of interatomic distances is provided.

Organometallic Uranium(V)−Imido Halide Complexes: From Synthesis to Electronic Structure and Bonding

Reaction of (C5Me5)2U(=N-2,4,6-(t)Bu3-C6H2) or (C5Me5)2U(=N-2,6-(i)Pr2-C6H3)(THF) with 5 equiv of CuX(n) (n = 1, X = Cl, Br, I; n = 2, X = F) affords the corresponding uranium(V)-imido halide complexes, (C5Me5)2U(=N-Ar)(X) (where Ar = 2,4,6-(t)Bu3-C6H2 and X = F (3), Cl (4), Br (5), I (6); Ar = 2,6-(i)Pr2-C6H3 and X = F (7), Cl (8), Br (9), I (10)), in good isolated yields of 75-89%. These compounds have been characterized by a combination of single-crystal X-ray diffraction, (1)H NMR spectroscopy, elemental analysis, mass spectrometry, cyclic voltammetry, UV-visible-NIR absorption spectroscopy, and variable-temperature magnetic susceptibility. The uranium L(III)-edge X-ray absorption spectrum of (C5Me5)2U(=N-2,4,6-(t)Bu3-C6H2)(Cl) (4) was analyzed to obtain structural information, and the U=N imido (1.97(1) A), U-Cl (2.60(2) A), and U-C5Me5 (2.84(1) A) distances were consistent with those observed for compounds 3, 5, 6, 8-10, which were all characterized by single-crystal X-ray diffraction studies. All (C5Me5)2U(=N-Ar)(X) complexes exhibit U(V)/U(IV) and U(VI)/U(V) redox couples by voltammetry, with the potential separation between these metal-based couples remaining essentially constant at approximately 1.50 V. The electronic spectra are comprised of pi-->pi* and pi-->nb(5f) transitions involving electrons in the metal-imido bond, and metal-centered f-f bands illustrative of spin-orbit and crystal-field influences on the 5f(1) valence electron configuration. Two distinct sets of bands are attributed to transitions derived from this 5f(1) configuration, and the intensities in these bands increase dramatically over those found in spectra of classical 5f(1) actinide coordination complexes. Temperature-dependent magnetic susceptibilities are reported for all complexes with mu(eff) values ranging from 2.22 to 2.53 mu(B). The onset of quenching of orbital angular momentum by ligand fields is observed to occur at approximately 40 K in all cases. Density functional theory results for the model complexes (C5Me5)2U(=N-C6H5)(F) (11) and (C5Me5)2U(=N-C6H5)(I) (12) show good agreement with experimental structural and electrochemical data and provide a basis for assignment of spectroscopic bands. The bonding analysis describes multiple bonding between the uranium metal center and imido nitrogen which is comprised of one sigma and two pi interactions with variable participation of 5f and 6d orbitals from the uranium center.

Actinide Selective Systems for Environmental Extraction and Sensing Applications

AbstractThe potential environmental and health concerns surrounding actinides and the use of nuclear fuels limits the acceptance of nuclear power by the public. This in turn, hinders the capability of this country to take advantage of nuclear power. Expanding our fundamental knowledge of actinide coordination chemistry will allow for the development of improved actinide sensors, new separations methods, or new means of radioactive waste remediation. We have designed and optimized a solution-phase parallel method for the synthesis of a library of symmetrical 2-quinoxalinol salens, Schiff-base type ligands with a 2-quinoxalinol incorporated into the salen backbone. This combines the rigid salen coordination framework with the quinoxaline properties that impart properties for use in colorimetric or fluorescent sensors. These have now been incorporated into organic soluble resins for metal extraction.

On the Suitability of Lanthanides as Actinide Analogs

AbstractWith the current level of actinide materials used in civilian power generation and the need for safe and efficient methods for the chemical separation of these species from their daughter products and for long-term storage requirements, a detailed understanding of actinide chemistry is of great importance. Due to the unique bonding properties of the f-elements, the lanthanides are commonly used as structural and chemical models for the actinides, but differences in the bonding between these 4f and 5f elements has become a question of immediate applicability to separations technology. This brief overview of actinide coordination chemistry in the Raymond group at UC Berkeley/LBNL examines the validity of using lanthanide analogs as structural models for the actinides, with particular attention paid to single crystal X-ray diffraction structures. Although lanthanides are commonly accepted as reasonable analogs for the actinides, these comparisons suggest the careful study of actinide materials independent of their lanthanide analogs to be of utmost importance to present and future efforts in nuclear industries.

Combining theoretical chemistry and XANES multi-edge experiments to probe actinide valence states

Abstract Both structural and electronic properties of the actinide cations are of fundamental interest in order to describe the intramolecular interactions. The 5f and 6d orbitals are the first partially or totally vacant states of these elements and their properties reflect the nature of the actinide–ligand bond. Because of its chemical and orbital selectivities, XANES spectroscopy is useful to probe the actinides' frontier orbitals and then understand the cation reactivity toward chelating ligands. The actinide L 3 edge contains structural information on the coordination polyhedron because of important scattering features. But very little electronic information can be extracted, due to the short core-hole lifetime, broadening the edge signal. On the other hand, the actinide M 4,5 edges provide a better resolution and allow one to achieve electronic and structural information. Furthermore, coupling simulations of the experimental spectra and quantum chemical calculations lead to quantitative information such as the determination of the actinide coordination sphere and its effective charge.

Studying Actinide Correlations in Solution using High-Energy X-ray Scattering

AbstractHigh energy x-ray scattering (HEXS) is becoming a valuable tool for quantifying actinide coordination environments in solution. The technique is described with and exemplified by a study on Th speciation. A comparison of data from two samples with pHs of 0 and 3.4 show differences in the Th structural environment that can be attributed to Th colloid formation at the higher pH. A further comparison of the HEXS data from the colloidal sample with Th L3-edge EXAFS data on the same sample clearly demonstrates the advantage of the scattering data when looking for correlations at second-near neighbor or longer distances.

Redox Chemistry of Actinide Ions in Wells−Dawson Heteropolyoxoanion Complexes

AbstractThe redox behavior has been characterized for several actinide (An) complexes with the monovacant Wells−Dawson anion, of the form [Ann+(α‐2‐P2W17O61)2]n−20 (An = Th4+, U4+, Np4+, Pu4+, and Am3+). Two complexes, with An = U4+ and Am3+, show redox activity under oxidizing conditions, which is attributed to the actinide oxidation. Am3+ is oxidized to Am4+ with an E1/2 = +1.21±0.01 V, and U4+ oxidizes to U5+ with a measured E1/2 = +0.55±0.01 V vs. Ag/AgCl. Although the cyclic voltammetry (CV) data are consistent with a reversible redox couple, bulk oxidative electrolysis of [U4+(α‐2‐P2W17O61)2]16− results in the decomposition of this complex to produce uranyl acetate and the free monovacant Wells−Dawson anion. In contrast, all of the CV data from the actinide coordination complexes differ from equivalent data obtained from the [α‐2‐P2W17O61]10− ligand itself. There are two complexed An4+ ions, Np and Pu, that undergo reduction over the same potential range as the ligands themselves. In situ X‐ray spectroelectrochemistry is used to quantify the actinide response. The Np4+/Np3+ redox behavior is a classically single ion process, with a formal potential of −0.84±0.01 V that was determined from a Nernst plot of X‐ray absorption near‐edge structure (XANES) data. The Pu4+/Pu3+ formal reduction potential in the complex [Pu(α‐2‐P2W17O61)2]n− was determined to be −0.17±0.01 V using the same methodology. However, in this latter case, the slope of the Nernst plot indicates that 0.72±0.03 electrons are involved in the reduction. This is a significant deviation from the 1 electron expected for the Pu couple, and is discussed in terms of the concomitant reduction of the P−W−O framework of the Wells−Dawson anion. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2003)

Domains of Actinide Atoms in Crystals of Fluorine-Containing Compounds

The Voronoi-Dirichlet polyhedra of actinide atoms A (A = Th, Pa, U, Np, Pu) in the crystal structure of fluoride-containing compounds were analyzed. It was found that the volumes of the Voronoi-Dirichlet polyhedra of actinide atoms surrounded by fluorine atoms depend on the actinide valence state and are practically independent of the actinide coordination number. It was shown by an example of the coordination polyhedra AOn and AFn that the geometric parameters of the Voronoi-Dirichlet polyhedra of actinide atoms and of the nonmetal atoms surrounding them are interrelated. In particular, increase in the electronegativity from oxygen to fluorine decreases the size of A atom irrespective of its oxidation state, on the average by 0.05 A.

Actinide coordination sphere in various U, Np and Pu nitrato coordination complexes

Waste management of nuclear fuel represents one of the major environmental concerns of the decade. To recycle fissile valuable materials, intimate knowledge of complexation mechanisms involved in the solvent extraction processes is indispensable. Evolution of the actinide coordination sphere of AnO2(NO3)2TBP-type complexes (An = U, Np, Pu; TBP = tributylphosphate) with the actinide valence state have been probed by XAS at the metal L III edge. Dramatic changes in the actinide coordination sphere appeared when the An(VI) metal is reduced to An(IV). However, no significant evolution in the actinide environment has been noticed across the series UO2 2+, NpO2 2+ and PuO2 2+.

EELS analysis of redox in glasses for plutonium immobilization

The chemical and structural environments of f-electron elements in glasses are the origin of many of the important optical, electronic, and magnetic properties of materials incorporating these elements. Thus, the oxidation state and chemical coordination of lanthanides and actinides in host materials constitute an important design consideration in optically active glasses, magnetic materials, perovskite superconductors, and nuclear waste materials. We have made use of the characteristic line shapes of cerium to determine its oxidation state in alkali borosilicate glasses that are being developed for immobilization of plutonium. Cerium, it should be noted, is often used as a “surrogate” element for plutonium in materials design because of its similar ionic size (for Pu in the + 3 and + 4 states) and preferred chemical coordination. The solubility of the plutonium (or cerium) in a waste glass will likely be determined by its redox state in the glass. By examining several compositions of prototype immobilization glass using electron energy loss spectroscopy (EELS), we found that the redox state of cerium doped to 7 wt% could be varied by a suitable choice of alkali elements in the glass formula. Preliminary results on plutonium-doped glasses confirm the design strategy employed, leading to 5 wt% (or more) plutonium being truly dissolved in the glass.

Coordination of Actinides in Silicate Melts

The structural environments around Th(IV) and U(VI) at concentrations ranging from 90 ppm to 7 wt.% were investigated in glasses and melts of Na di- and trisilicate compositions between 293 and 1550 K using x-ray absorption fine structure (XAFS) spectroscopy. Data for model compounds were collected at temperatures up to 2000 K in order to quantify the magnitude of anharmonic effects. Data for glasses and melts were collected and analyzed considering anharmonic (cumulant-expansion), curved-wave and multiple-scattering effects. In the glasses and melts studied, U(VI) occurs as uranyl groups [d(U(VI)-O 1.78 and 2.25(3) A] and Th(IV) occurs as a mixture of 6- and 8-coordinated Th at the 1-3 wt.% Th level, but as 8-coordinated Th at trace levels (90-1000 ppm). These two actinides do not undergo significant coordination changes during the glass-to-melt transformation. However, special care in the data analysis protocol is needed to avoid the effects of anharmonicity (due to both positional and thermal effects) which tend to artificially increase the (apparent) amount of 6-coordinated Th.

F-Element complexation by diphosphonate ligands

To develop a fundamental understanding of the nature of lanthanide and actinide coordination complexes with ligands bearing phosphonate groups, we have examined the kinetics, thermodynamics and spectroscopy of lanthanide and actinide complexes with phosphonate ligands in aqueous solutions and resolved crystal structures for selected lanthanide complexes. The combined results of two-phase distribution experiments, titration calorimetry, laser-induced fluorescence decay, and molecular mechanics modeling have been used to infer the. structure of 1:2 complexes of Eu(III) with gem-diphosphonic acids in aqueous solution. A complementary crystallographic and FTIR investigation of the structure of lanthanide complexes in the solid state indicates a general similarity between amorphous and crystalline complexes, differing in the relative hydration of the solid. There are no inner sphere water molecules in the solid state. Parallel studies of the kinetics of formation and dissociation of f-element diphosphonate complexes demonstrate that the relative stability of these complexes is governed primarily by differences in the rate of complex formation. A related investigation has established the existence of a correlation between complex formation rate and cationic radius which is independent of the charge on the cation.

The “Regular” and “Inverse” Tetrad Effect

A correlation between the direction of the tetrad (double-double) effect in the lanthanide and actinide series and coordination number (CN) is discussed on experimental and theoretical grounds. Some examples of determining the CN values of the f-block elements in solution complexes using the tetrad effect are reviewed. Except for the CN implication, the tetrad effect of the inner transition elements is presented in terms of effective crystal radius, covalent shortening, nephelauxetic and ligand field effects, spin-orbit coupling, hydration, etc.